1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate, which is developed using radiation. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate or by etching.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. This measurement may take place during the lithographic process, or separately from it, but is usually carried out using a separate metrology apparatus from the lithographic apparatus, as each apparatus involves a not insignificant amount of relative specialism.
The measurement and inspection step after development of the resist, or substrate surface in the case of etching, is referred to as in-line because it is carried out in the normal course of processing production substrates, and typically serves two purposes. First, it is desirable to detect any target areas where the pattern in the developed resist is faulty. If a sufficient number of target areas are faulty, the substrate may be stripped of the patterned resist and re-exposed, hopefully correctly, rather than making the fault permanent by carrying out a process step, e.g., an etch, with a faulty pattern. Second, the measurements may allow errors in the lithographic apparatus, e.g., illumination settings or exposure dose, to be detected and corrected for in subsequent exposures.
However, many errors in the lithographic apparatus may not easily be detected or quantified from the patterns printed in resist. Detection of a fault does not always lead directly to its cause. Thus, a variety of off-line procedures for detecting and measuring errors in the lithographic apparatus are known. These may involve replacing the substrate with a measuring device or carrying out exposures of special test patterns, e.g., at a variety of different machine settings.
There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate, or the structures on the substrate, can be determined. A structure on the substrate that gives rise to a reflected spectrum may be reconstructed, e.g., using real-time regression or by comparison to a library of patterns derived by simulation. Reconstruction involves minimization of a cost function. Both approaches calculate the scattering of light by periodic structures. The most common technique is Rigorous Coupled-Wave Analysis (RCWA), though light scattering may also be calculated by other techniques such as Finite Difference Time Domain (FDTD) or Integral Equation techniques.
Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum, intensity as a function of wavelength, of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
One parameter on a target on the substrate surface that needs to be measured is overlay. Overlay is the offset of a structure on one substrate layer with respect to a structure on an earlier layer (i.e., a lower layer or a layer closer to the substrate). If there is an overlay, the overall structure after exposure of all the layers will not be formed accurately and may cause problems for the resulting product. An overlay is measured by inspecting the symmetry of the overall stack or structure. Overlay metrology is based on the measurement of an asymmetry in the angular scatter spectrum. Symmetric structures yield symmetric angular spectra and an asymmetry in the target shows up as an asymmetry in the angular scatter spectrum. This property is the basis of overlay metrology using angle-resolved scatterometry.
The radiation used for the overlay metrology is typically a circular or annular beam. An annular beam is used rather than a circular beam because the overlap in the resultant scattered spectrum of the zeroth order diffraction spectrum with the +first, and potentially higher, diffraction orders is easier to decipher with annular radiation beams and fewer of the available photons are “wasted” or “lost”. As it is the parts of the diffraction orders that do not overlap that give the information, the overlapping parts are not used for measurement and are therefore “wasted”. Only the first “free order” (i.e., the portion of the radiation that does not overlap) contains useful information about the overlay. However, even using an annular radiation beam may not prevent some of the beam from being lost because as targets get smaller, parts of the annular beam should be discarded if it contains information from neighboring targets or even any off-target surface. The beam may not simply be shrunk to fit smaller target sizes, as information from higher diffraction orders is likely to be lost.